1. Technical Field
This invention relates to a range finder for passive type autofocussing device so arranged that light rays emitted from a scene to be photographed are picked up to find a range to said scene and the objective is adjustably brought into focus based on a result of said range finding.
2. Prior Art
The autofocussing device is used to find a shooting range for photographic camera or the like in automatic mode and to bring the objective into focus based on a result of the range finding and such autofocussing device allows everyone to enjoy photographing easily. Various types of autofocussing device have already been developed and most of them employ the trigonometrical range finding method. Typical autofocussing device relying on this trigonometrical range finding method is so-called passive type autofocussing device adapted to pick up light rays emitted from the scene by photosensors provided on the camera and thereby to find a shooting range.
Some of the passive type autofocussing devices include a pair of photosensors. However, if the scene includes two objects being in contrast with each other, such range finder provided with a pair of photosensors disadvantageously indicates two different states of the single scene to be photographed and consequently can not achieve a reliable range finding, necessarily resulting in a picture which is out of focus.
To assure a reliable range finding and thereby to obtain a well-focused picture, the applicant of this application has previously proposed a range finding mechanism comprising three photodetector arrays (Japanese Patent Application No. 1989-177382, Japanese Patent Application Disclosure No. 1991-42642). A principle of range finding by this range finding mechanism will be described in reference with FIGS. 23 and 24 of the attached drawing. The range finding mechanism comprises a reference photosensor 1, a first photosensor 2 and a second photosensor 3. These photosensors 1, 2, 3 comprise, in turn, imagining lenses 1a, 2a, 3a and photodetector arrays 1b, 2b, 3b, respectively, so that a scene to be photographed is imaged through the imagining lenses 1a, 2a, 3a on the photodetector arrays 1b, 2b, 3b, respectively. FIG. 23 illustrates a case in which the scene P comprises a single object. Now, referring to FIG. 23, x.sub.0 represents a displacement of an output signal P.sub.0 relating to a luminance distribution of the object P detected by the reference photodetector array 1b with respect to an optical axis T.sub.0 of the reference photosensor 1, x.sub.1 represents a displacement of an output signal P.sub.1 relating to a luminance distribution on the object P detected by the first photodetector array 2b with respect to an optical axis T.sub.1 of the first photosensor 2, and x.sub.2 represents a displacement of an output signal P.sub.2 relating to a luminance distribution on the object P detected by the second photodetector array 3b with respect to an optical axis T.sub.2 of the second photosensor 3. These displacements x.sub.0, x.sub.1, x.sub.2 represents phase differences relating to the luminance distribution on the object detected by the photodetector arrays 1b, 2b, 3b, respectively. Assume that the optical axes T.sub.0, T.sub.1, T.sub.2 are spaced from one another by B, photodetective surfaces of the photodetector arrays 1b, 2b, 3b are spaced from the respective imaging lenses 1a, 2a, 3a by A, and the object P lies at a distance Lp from the imagining lenses 1a, 2a, 3a and at a distance X from the optical axis T.sub.0, the following equation is derived from the principle of trigonometrical survey: EQU X=x.sub.0 *Lp/A (1).
If a direction in which the output signal image appears with respect to the optical axis T.sub.0 is taken into account, EQU -x.sub.1 =(B-X)/Lp*A (2) EQU x.sub.2 =(B+X)/Lp*A (3).
If the equation (1) is substituted for these equations (2), (3), respectively, EQU x.sub.1 =(B/Lp)*A+x.sub.0 ( 4) EQU x.sub.2 =(B/Lp)*A+x.sub.0 ( 5).
Comparison of the equations (4) and (5) indicates that x.sub.1 and x.sub.2 are displaced with respect to a reference x.sub.0, respectively, by an amount EQU (B/Lp)*A=Xp (6).
Accordingly, this Xp may be obtained to compute EQU Lp=A*B/Xp (7).
Procedure used to obtain said Xp will be explained in reference with FIG. 24. Referring to FIG. 24, (a) illustrates output signals relating to the luminance distribution detected by the photodetector arrays 1b, 2b, 3b exposed to light rays emitted from two objects with respect to reference output signals P.sub.0, Q.sub.0. From the state of (a), the output signal waveforms P.sub.1, P.sub.2 may be shifted with respect to said output waveform P.sub.0 until these output signal waveforms P.sub.0, P.sub.1, P.sub.2 coincide with one another to obtain an amount of said displacement Xp. More specifically, at this moment of coincidence, P.sub.1 and P.sub.2 have been displaced by an equal amount. Accordingly, when said three output signal waveforms coincide with one another after the output signal of the photodetector array 2b and the output signal of the photodetector array 3b have been shifted by an equal amount, the waveforms of these three output signals will provide the data relating to the same object P. Next, as illustrated by (c), the output signal Q.sub.1, Q.sub.2 may be shifted with respect to the output signal Q.sub.0 until said output signal Q.sub.1, Q.sub.2 coincide with the output signal Q.sub.0 to obtain an amount of the displacement Xq.
Based on said Xp, Xq obtained in the manner as has been described above, the ranges Lp, Lq to the objects P, Q, respectively, are computed according to said equation (7).